Various types of radiation are utilized for applications in industry, medicine, and other areas. Such radiation may include neutrons, gamma rays, and X-rays, each of these types of radiation having certain advantages and disadvantages when used for various applications. One frequently used source for such radiation is an accelerator type generator which generates the desired radiation by bombarding a suitable target with accelerated high energy charged particles. In such devices, it is frequently desired to obtain a very high radiation yield or flux. This normally requires that the charged particles impinging on the target be at high energy. To accelerate the charged particles to such high energy, to the extent that it is currently possible, generally requires large and expensive equipment. Accelerators capable of generating high energy charged particles are also required in applications other than radiation sources. A need therefore exists for an accelerator capable of producing very high energy charged particles, for example ions, protons, or electrons, which is relatively compact and inexpensive. Such device should also be reliable, having a reasonable mean time between failures.
One type of radiation which is becoming increasingly useful in commercial applications is neutrons. Most such applications, (which include explosive detection, such as mine detectors and baggage inspection devices for airports or other transportation facilities, drug detection and nondestructive testing of metals and other dense materials) require thermal neutrons with a high neutron flux. For explosive detection, advantage is taken of the fact that, when a thermal neutron flux is directed at a high explosive, such as that contained in a mine, there is a neutron capture reaction in the nitrogen component of such explosive which results in high energy 10.8 MeV gamma rays being produced. These gamma rays provide a unique signature for the nitrogen component and may be detected to indicate the presence of explosives.
While thermal neutrons are preferred for applications such or as those indicated above because they interact more strongly, and therefore provide a more easily discriminatable signature, fast neutrons may also be utilized for applications such as drug or explosives detection by detecting the unique scatter cross section of the neutrons, gamma ray emissions caused by such neutrons or both. Fast neutrons may be preferable in some such applications because of their capability of penetrating deeper and the fact that they are less easily shielded. The deeper penetration capability may, for example, make fast neutrons preferable for mine detection applications.
Current devices for detecting explosives in this way utilize a radioisotope, for example a 252 Cf radioisotope as the neutron source. However, a radioisotopic neutron source cannot be switched off when not in use. Since radiation is potentially hazardous, the inability to switch the device off has resulted in various methods being utilized for shielding the radioisotope source when the source is not in use. This need for shielding results in at least three potential problems. First, the shielding is cumbersome for use in the field, such as would normally be required for explosives detection, and adds significantly to the system size, weight and complexity. In addition, the 252 Cf source, which has a half life of 2.5 years, creates logistic burdens associated with its acquisition, storage, replacement and disposal. Finally, to reduce shielding requirements within commercially acceptable limits, and to reduce the radiation hazard, such sources currently in use generally produce a relatively low neutron yield which is barely adequate for the explosives detection application, for example neutron yields up to 2.times.10.sup.8 neutrons/second. This limits the scanning velocity at which the systems utilizing such source can be operated. Such yields are not adequate for various other nondestructive testing applications which may require yield in the 10.sup.11 -10.sup.12 range.
For these reasons, a reliable electronic neutron generator, such as an accelerator neutron source, would offer significant advantages. These advantages would include on/off switchability, no persistent radioactivity (except for minute induced radioactivity in material inside the generator which should have a half life of only a few minutes) and both higher and controllable neutron flux. In particular, neutron fluxes which may range up to the 10.sup.12 neutron/second range are possible. However, existing electronic neutron sources have been large and costly and have not had adequate reliability-y for explosive or drug detection or nondestructive testing applications. For example, one available neutron generator capable of providing the required neutron flux operates at a relatively high deuteron current, approximately 4 mA, which results in limited source life due to the high power loading on the target and sputtering from the target and other surfaces within the accelerator. A higher energy device utilizing a Van de Graaff generator is capable of generating very high neutron fluxes and operates at low beam current and power, thus providing an extended operating lifetime. However, this improvement in life is achieved at the expense of greatly increased system size and cost, the size and cost of this device making it unsuitable for application as the neutron source in most explosive detection applications.
Similar problems arise with neutron sources utilized for nondestructive testing of aircraft parts or other objects fabricated of metal or other dense materials, where X-rays cannot provide sufficient penetration. Such testing is utilized to locate hidden cracks or other defects in the material. With the increasing age of both military and commercial aircraft, and recent accidents resulting from metal fatigue and related problems, the need for a relatively effective and inexpensive device for nondestructively testing aircraft parts is apparent. A similar situation exists with invisible parts, such as metal supports in bridges and other structures, where nondestructive testing is required.
In view of the above, it is apparent that a need exists for a relatively compact and inexpensive radiation source, and in particular a neutron source which produces a high yield of neutrons (i.e., in the 10.sup.8 to 10.sup.12 neutrons/second) range, which is reliable, can operate for a relatively long period of time (greater than 2000 hours mean time between failures), may have its radiation output level easily controlled, and may be switched on and off so as to not generate harmful radiation when not in use. A need also exists for various explosive and drug detection devices and for nondestructive testing systems employing such a neutron source.